If the conditions are changed after equilibrium has been established, the system may no longer be at equilbrium and may move in one direction or another to re-establish equilibrium. The direction in which the system will move to re-establish equilibrium can be predicted by Le Chatelier's principle:
Le Chatelier's Principle
If a factor affecting the position of an equilibrium is altered, the position of the equilibrium shifts to oppose the effect of the change.
Such constraints can be the addition or removal of one of the reactants or products, a change in pressure, a change in temperature or the addition of a catalyst.
Each must be treated separately:
1. Concentration
- Le Chatelier's principle predicts that if a reactant's concentration in a system is increased, the system will move to the right in order to decrease the concentration of that reactant.
- If the reactant's concentration is decreased, the system will move to the left in order to replace that reactant.
- Similarly, if a product's concentration is increased then the system will move to the left.
- If a product's concentration is decreased then the system will move to the right.
Changing the concentrations of reactants and products has no effect on the rate constants of the forward or reverse reactions. Therefore it has no effect on the equilibrium constant.
Given that Kc is unchanged, the effect of changing the concentration of one of the species can be shown with reference to Kc:
Consider the reaction: aA + bB ⇌ cC + dD
Kc = [C]c [D]d
[A]b [B]b
If the concentration of A or B is increased, then [C] and [D] must increase to maintain Kc.
Similarly if the concentration of C or D is increased, then [A] and [B] must increase to maintain Kc.
2. Pressure
The pressure in a system depends on the number of gas molecules in the system. Le Chatelier's principle therefore predicts that if the pressure of the system is increased, the system will move towards the side which has fewer gas moles. If the pressure of the system is decreased, the system will move towards the side which has more gas moles. If the number of gas moles on both sides in the same, then pressure has no effect on the equilibrium position.
- Eg: PCl5(g) ⇌ PCl3(g) + Cl2(g)
If the pressure is increased, the system will move to the left. If the pressure is decreased, the system will move to the right.
- Eg: 2SO2(g) + O2(g) ⇌ 2SO3(g)
If the pressure is increased, the system will move to the right. If the pressure is decreased, the system will move to the left.
- Eg: 2H2S(g) + SO2(g) ⇌ 3S(s) + 2H2O(l)
If the pressure is increased, the system will move to the right. If the pressure is decreased, the system will move to the left.
- Eg: H2(g) + I2(g) ⇌ 2HI(g)
Changing the pressure will have no effect on the position of this equilibrium.
Since changes in pressure affect the reactant concentrations and not the rate constants, it follows that the equilibrium constant is not affected by changes in pressure.
This is a lot of information, take your time to understand the concept, as this always appear in AS/A-level Chemistry exams! 👩🏫 You have done great so far!
3. Temperature
- If the forward reaction is exothermic, then the temperature of the system will rise if the forward reaction takes place. The reverse reaction will therefore be endothermic, and the temperature of the system will fall if the reverse reaction takes place.
- Le Chatelier's principle therefore predicts that an increase in temperature will favour the endothermic reaction.
- A decrease in temperature will favour the exothermic reaction.
- If the forward reaction is exothermic, then an increase in temperature will cause the system to shift to the left, and a decrease in temperature will cause the system to shift to the right.
- If the forward reaction is endothermic, than an increase in temperature will cause the system to shift to the right, and a decrease in temperature will cause the system to shift to the left.
- If DH = 0, then a change in temperature will have no effect on the position of equilibrium.
In AS/A-level Chemistry exams, you are required to give explanations like the examples below!👨🏫
- Eg: 2SO2(g) + O2(g) ⇌ 2SO3(g), DH = -ve
The forward reaction is exothermic so is favoured by decreasing the temperature. The reverse reaction is endothermic so is favoured by increasing the temperature.
- Eg: CaCO3(s) ⇌ CaO(s) + CO2(g), DH = +ve
The forward reaction is endothermic so is favoured by increasing the temperature. The reverse reaction is exothermic so is favoured by decreasing the temperature.
A change in temperature changes the rate constants for both forward and reverse reactions and is therefore likely to change the value of the equilibrium constant. If the reaction is exothermic, then an increase in temperature will cause the value of Kc to decrease, and if the reaction is endothermic, then an increase in temperature will cause the value of Kc to increase.
4. Catalysts
The addition of a catalyst will have no effect on the position of equilibrium. It will increase the rate of the forward and reverse reactions, but by the same amount. The position of equilibrium will thus be unchanged.
As the position of equilibrium is unchanged, it follows that adding a catalyst has no effect on the equilibrium constant.
Reference:
https://getrevising.co.uk/resources/energetics
This is the end of the topic!
Drafted by Cherry (Chemistry)